Permeable base transistors (PBT) made with either GaAs or silicon and having an embedded base grid structure have been proposed.
The PBT structure has at least one electron conduction channel and a metal base which pinches off the conduction channel without any voltage applied to the base. The conduction channel is opened and controlled by applying a forward voltage with respect to the emitter at the metal base. In order to achieve high operation frequency, the length of the channel region which is affected by the applied bias voltage must be as short as possible to have a short electron transit time. To meet this goal, the following criteria should be met:
(1) The doping in the conduction channel has to be high enough to cause a thin depletion region.
(2) Due to the high doping requirements of the first condition, the grid spacing "d" between the base has to be small enough (in the submicron region) to pinch off the channel at zero bias.
(3) The thickness of the metal base region "L" should be less than half the grid spacing "d", and "d" is less than the zero bias depletion width A.sub.O associated with the base region.
(4) The spacing "d" between the base region should be about 200 nanometers or less.
Previously, PBT processing has required x-ray lithography to defines a base spacing of 200 nanometers, deposition of a 20nm tungsten grid for the base metal, and the subsequent epitaxial overgrowth of GaAs or Si over the base grid to fabricate the emitter region. These processing steps are rather complex and are not yet suitable for low cost volume production. It would be advantageous to provide a simpler method to fabricate submicron structures in silicon that would allow silicon PBT's and other semiconductor devices to be made without the need of a crystal overgrowth over a patterned metal grid.